Abrupt change reflected by the van Hove singularity in the optical spectra of CaMn2P2

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The trigonal CaAl2Si2-type structure CaMn2P2 has been reported undergoing an exotic first order phase transition at the critical temperature T K. In this paper, we present the optical spectra for the ab-plane of CaMn2P2 single crystal from 300 K to 10 K for the first time. In the real part of the optical conductivity spectra σ 1(ω), a direct gap could be determined at all temperatures without any Drude term visible, i.e., the sample goes through the first order phase transition from one insulator state to the other insulator state. At higher energy, an asymmetric sharp interband transition peak appears in all σ 1(ω) spectra, which indicates a divergence of the joint density of states. This sharp peak could be well described by the two dimensional van Hove singularity function. In particular, this peak is very sensitive to the first order phase transition, especially the peak position E whose the most prominent blue shift occurs only when the first order transition happens. Our data and analysis reveal that the first order phase transition leads to a weak partial re-normalization of the band structure. Our study will be useful in further investigations about the mechanism of the first order phase transition in the insulator.